Recently, metal halide lamps have been actively developed and promoted, using a tube made of translucent polycrystalline alumina ceramic (PCA) in place of quartz as an arc tube material. Although quartz has a heat resistance up to about 1,000° C., the PCA tube has a higher heat resistance up to about 1,200° C., thereby allowing the tube wall load to be set in a higher range. Therefore, the use of the PCA tube makes it possible to provide a metal halide lamp with a higher lamp efficiency. At first, metal halide lamps (ceramic metal halide lamps) using the PCA tube were developed and commercialized for indoor lighting applications, such as in shops, which require a high color rendering. Recently, high efficiency metal halide lamps have been under development for general outdoor lighting applications.
With respect to high efficiency metal halide lamps for outdoor lighting applications using a conventional quartz arc tube, there has been a proposal to use, as light emitting substances, a combination of sodium halide (NaX) and a halide of a lanthanoid rare-earth metal, such as cerium halide (CeX3) and praseodymium halide (PrX3), which emit light whose spectrum is in a wavelength region of high spectral luminous efficiency (see Japanese Laid-Open Patent Publication No. Sho 57-92747, for example).
Meanwhile, with regard to a recently developed ceramic metal halide lamp, there has been a proposal to use cerium iodide (CeI3) and sodium iodide (NaI) as light emitting substances. This metal halide lamp has a high efficiency, for example, 115 lm/W with a 300 W type. It also has a sufficient color rendering for outdoor lighting, with a color rendering index Ra of 70 and a rated life time of 12,000 hours.
Also, a conventional metal halide lamp is structured such that its arc tube is contained in an outer tubular bulb of, for example, a drop shape, and that an inert gas such as nitrogen (N2) is sealed in the outer tubular bulb. The arc tube is held by stem leads and the like in the outer tubular bulb.
A higher efficiency ceramic metal halide lamp using the above-mentioned light emitting substances (CeI3, NaI) has also been proposed (see Japanese Unexamined Patent Publication No. 2000-501563, for example). In this metal halide lamp, the tubular shape parameter (the ratio of the electrode interspacing Le to the internal diameter φi of the central tubular part) is greater than 5, and the arc tube is relatively long and narrow. Also, the molar ratio of the light emitting substances: NaI/CeI3 is 3 to 25, and the tube wall load (we) is set to 30 W/cm2 or lower. This construction provides a high lamp efficiency of 130 lm/W with a 150 W type, and a color rendering of Ra 53, for example.